SWA armour supply to outbuilding, what CSA cable? and Reference Method?

[Sparky_marky2]

Hi guys,

I am running a supply to an outbuilding that requires a 22m run from the Main CU (in the garage) to the CU in the outbuilding, at the Main CU it will be a 40A MCB supplying an 63ma RCD protected sub board which will then supply:

6A type B MCB for 12 downlights and an extractor fan
32A type B MCB for 3 double sockets
20A type B MCB for a 3.5kw split air condition unit

I've calculated the voltage drop if using 6mm 3 core SWA clipped direct (table 4E4A p372 BS 7671) 53 x 40A x 22m / 1000 = 46.64 Vdrop .. thats a crazy figure

And the supply will be feeding a sub board initially based on what I've been taught I calculated maximum demand by adding the breakers amp size and x them by 40% so 6+32+20 x 40% = 23.2A

The SWA will run up the outside wall from the garage into the loft and out the other end, and down galvanised conduit, into the ground and into and up a wall with insulation (2m) and into the outbuilding

The queries I have are:

The majority of the run will be clipped direct and only a small part of the length will be in insulated brick wall am I right/wrong to assume Reference method C (clipped direct)?

And as it is SINGLE PHASE 3 CORE cable I'm using I'm a little confused as the only two options under clipped direct in table 4E4A (p372 BS 7671)is '3 CORE 3 phase AC' and '1 two core cable SINGLE PHASE AC' my installation does not fall perfectly into either option, which should I select?

And how best would you recommend lowering the Voltage drop to the stipulated 5% 11.5V?


Thank you in advance for helping,

Mark

 

[telectrix]

He's used the CCC for 3 or 4 core 6.0mm from table 4E4A....53a!
That would rather skew the result.

tables are useful only if you know how to use them.

 

[Pete999]

tables are useful only if you know how to use them.

My kitchen table has a wonky leg, but it's easy to use if you know how.

 

[telectrix]

i sympathise with your kitchen table. i got 2 wonky legs (even when sober).

 

[Pete999]

i sympathise with your kitchen table. i got 2 wonky legs (even when sober).

Yes, but that's self inflicted, mind you I was wobbly this morning, to much vino

 

[Sparky_marky2]

Hi guys, thank for you for all the helpful posts! it was late lastnight when i was posting so i was looking at the wrong page for the Vdrop and actually looking at the current carrying capacity as Davesparks pointed out.

But this has now raised up another related issue about voltage drop with the sub board distribution cable, I've calculated that i need to up it to 10mm SWA for such a run of 22m but also there's the combined Vdrop from the circuits on the sub-board that i need to account for EG. the Vdrop for the length of the circuit from the Main CU to the Sub-board + the Vdrop from the Sub-board MCB to the end of the lighting circuit on that board.. How do you combine two different voltage drops? (ie the lighting circuit must be within 3% and the distribution circuit must be within 5%, or do you treat thee two circuits independently?
likewise for the ring circuit on the sub-board they would both be 5% do they have to both add up to be less than 5% or is there another figure to go from?

any insight would be greatly appreciated.

Something I do regularly where an SWA drops down a wall into the ground. A meter or so of galv with a bushed top protects the cable from damage from strimmers and such like.

I'v chosen to run a part of the SWA in galv conduit as it will run past an outdoor step and be about 2 inches from the ground just for added mechanical protection from people kicking it or from some garden tools hitting it (just trying to account for worst possible situation)

Is this the same house as your other thread.

my other thread on here about alternate cables for external use is for the same house but is not related to the sub-board/sub-distribution cable.

 

[Sparky_marky2]

Hi guys I've attached a photo, at the roof of the bungalow (i've marked this in red) is where the SWA will come out of and down the wall, where it is marked in blue is the area I am concerned maybe 'at risk' from kicks and other problems from people walking past. The client has stated that the bit in blue that runs horizontally in front of the step will be concreted over once his outhouse has been completed.

 

[Pete999]

Hi guys I've attached a photo, at the roof of the bungalow (i've marked this in red) is where the SWA will come out of and down the wall, where it is marked in blue is the area I am concerned maybe 'at risk' from kicks and other problems from people walking past. The client has stated that the bit in blue that runs horizontally in front of the step will be concreted over once his outhouse has been completed.

View attachment 37579

What size galv conduit are you intending to use? 10mm2 2 core it will be a tight job to run that in conduit.

 

[Sparky_marky2]

Pete999 - I'm going to spec the job again later today as things have changed somewhat from initially using 6mm to now using 10mm SWA, after reading everyone's advice I may leave out the galv conduit and run the cable clipped direct.

 

[telectrix]

calculate VD for the sub first. from my previous post, for 6mm this would be 6V assuming a 40A load ( which is a bit pessimistic. then, calc. the shed VD.. add them together.

 

[telectrix]

Pete999 - I'm going to spec the job again later today as things have changed somewhat from initially using 6mm to now using 10mm SWA, after reading everyone's advice I may leave out the galv conduit and run the cable clipped direct.

or just use some plastic duct. and i think 10mm is overkill.

 

[Toneyz]

What size galv conduit are you intending to use? 10mm2 2 core it will be a tight job to run that in conduit.

Let alone the bend.

 

[Sparky_marky2]

calculate VD for the sub first. from my previous post, for 6mm this would be 6V assuming a 40A load ( which is a bit pessimistic. then, calc. the shed VD.. add them together.

I've calculated the Vdrop for the 10mm SWA on 40A MCB from the main board at 22m length run which comes to 3.8VD (1.68% of the allowable 5%)

The issue is the circuits coming off the sub-board.. for instance I've calculated the 2 double sockets that the client wants needs a run of 2.5 twin and earth on 32A MCB and 12m length run which comes to 5.76VD (2.5% of the allowable 5%)

IF i add these two figures together 1.68% and 2.5% i get 4.18% which is just below the allowable 5% (11.5V)

Am i right to add it like this and then do the same for the other circuits (lighting VD + 10mm SWA VD and air con VD + 10mm SWA) on the sub-board or should I treat them all individually and not add the figure for the VDrop figure for the 10mm SWA

 

[telectrix]

personally, i add the voltages first, then work oyt the %. i'd not be too worried if the figure was over the 3% anyway. you got 10V to play with due to the silly 230V nominal voltage rubbish figure.

 

[Sparky_marky2]

personally, i add the voltages first, then work oyt the %. i'd not be too worried if the figure was over the 3% anyway. you got 10V to play with due to the silly 230V nominal voltage rubbish figure.

Thanks for the help, how do you know this figure? is this in the BS7671.. i just want to do things by the book etc

 

[Deleted member 9648]

The total volt drop from the origin of the installation must not exceed 3% for lighting. For example if your distribution circuit is expected to carry a maximum load of 40a then the volt drop of both distribution circuit and the furthest point of the lighting circuit combined must not exceed 3%. Therefore you will calculate the maximum volt drop on your lighting circuit and add it to the calculated voltage drop on the sub. In total it should not exceed 6.9v
Put another way if your lighting circuit will have a VD of 2v from the CU then your distribution circuit fully loaded will have a permitted volt drop of 4.9v.